As the two–photon absorption was considered in [1]}, we will not discuss it as in–depth as we will with the third–harmonic generation. Similarly to what was done in Fig. REF , we plot the real and imaginary parts of the two–photon absorption third–order susceptibility for a broadening small enough \(\left(0.05\,\mathrm {meV}\right)\)  such that each individual peak can be clearly observed. This plot is visible in Fig. REF , normalized by the peak value for the \(1\rightarrow 3d\)  transition at \(\sim 16.6\,\mathrm {meV}\) , along with a diagram representing the \(1s\rightarrow ns\)  and \(1s\rightarrow nd\)  transitions.
